In the oil and gas industry, induction logging tools are frequently used to provide an indication of the electrical resistivity of rock formations surrounding a borehole. Such information regarding resistivity helps engineers to ascertain the presence or absence of hydrocarbons. A typical induction logging tool includes a transmitter antenna and a pair of receiver antennas located at different distances from the transmitter antenna along the axis of the tool. The transmitter antenna creates electromagnetic fields in the surrounding formation. In turn, the electromagnetic fields in the formation induce an electrical voltage in each receiver antenna. Due to geometric spreading and absorption by the surrounding formation, the induced voltages in the two receiving antennas have different phases and amplitudes. Experiments have shown that the real and/or imaginary portions of phase and amplitude of the induced voltages in the receiver antennas indicate the resistivity of the formation. The average depth of investigation (as defined by a radial distance from the tool axis) to which such a resistivity measurement pertains is a function of the frequency of the transmitter and the distance from the transmitter to the mid-point between the two receivers. Thus, one may achieve multiple radial depths of investigation of resistivity either by providing multiple transmitters at different distances from the receiver pair or by operating a single transmitter at multiple frequencies.
Many formations are electrically anisotropic, a property which is generally attributable to fine layering during the sedimentary build-up of the formation. Hence, in a formation coordinate system oriented such that the x-y plane is parallel to the formation layers and the z axis is perpendicular to the formation layers, resistivities Rx and Ry in directions x and y, respectively, are substantially the same, but resistivity Rz in the z direction is different from Rx and Ry. The resistivity in a direction parallel to the plane of the formation (i.e., the x-y plane) is known as the horizontal resistivity, Rh, and the resistivity in the direction perpendicular to the plane of the formation (i.e., the z direction) is known as the vertical resistivity, Rv. The index of anisotropy, η, is defined as η=[Rv/Rh]1/2. The relative dip angle, θ, is the angle between the tool axis and the normal to the plane of the formation. Resistive anisotropy and relative dip angle each have effects on resistivity logging tool measurements, which may be measured using multi-axial antenna orientations.
The induction tool is typically placed within a borehole drilled into the formation. The borehole may be filled with fluid, sometimes called mud, which gives rise to undesired effects, termed borehole effects, in the logging data due to differences in resistivity between the mud and the formation. The borehole effects are often removed from logging data (i.e., the data are “borehole corrected”) using a database compiled for different formation models; however, when such a database presumes the borehole is occupied with oil-based mud (“OBM”), the corrections would result in inaccurate resistivity logs if used without OBM.
It should be understood, however, that the specific embodiments given in the drawings and detailed description do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.